Method for improving the strength characteristics of orthopedic prostheses

ABSTRACT

A method for improving the strength characteristics of orthopedic prostheses, such as femur necks, femoral stems, cotyloid elements, systems of the spinal column, including the step of: starting from a prosthesis made of metal, subjecting the prosthesis to a peening treatment to improve the surface hardness of the prosthesis and eliminate micro-scoring caused by the machining of machine tools, the method further including a subsequent step of performing a surface finishing treatment by vibration finishing.

TECHNICAL FIELD

The present invention relates to a method for improving the strengthcharacteristics of orthopedic prostheses, such as cotyloid elements,femur necks, femoral stems, systems of the spinal column and the like.

BACKGROUND ART

As is known, orthopedic prostheses, such as femur necks, femoral stems,cotyloid elements and systems of the spinal column, as well as otherprostheses, are currently made of metal, for example titanium, so as tobe very light and tough.

However, such prostheses are often subject to breakage because they haveparts that can be particularly stressed as a consequence of theirimplantation.

Moreover, such prostheses are provided by using machine tools that usecutting tools for machining and such cutting tools leave micro-scoringon the surface of the prosthesis which is apparently not visible to thenaked eye but constitutes initiation points of fatigue failure cracks.

In particular, the micro-scoring that is present on the surface of suchprostheses constitutes weak points that produce cracks that ultimatelylead to failure of the prosthesis, which may occur at any time in anapparently inexplicable manner.

In order to improve the fatigue resistance of the prosthesis, it ispossible to subject it to a peening treatment that allows compaction ofthe material, increasing its fatigue resistance, and elimination or inany case substantial reduction of the “dents” produced by machining,decreasing the risk of cracks and corrosion.

However, it should be noted that the peening treatment, althougheffective in itself, may entail an increase in roughness due to theprotrusions and hollows that it generates.

Moreover, the peening treatment causes a variation of a few microns inthe geometric dimensions of the treated part.

Moreover, the peening itself might create cracks if the parameters ofthe process are not calculated well as a function of the material used.

For example, titanium, the material usually used for prostheses, issubject to crack forming.

DISCLOSURE OF THE INVENTION

The aim of the present invention is to provide a method for improvingthe strength characteristics of prostheses and the like, substantiallyeliminating the likelihood that the micro-scoring caused by machiningwith machine tools might generate cracks that lead to the failure of theprosthesis.

Within this aim, an object of the present invention is to provide amethod for improving the strength of prostheses and the like that can beused after the standard production step of the prosthesis, thus withoutchanging substantially its production cycle.

Another object of the present invention is to provide a method forimproving the strength characteristics of prostheses and the like thatis highly reliable, relatively simple to provide and has competitivecosts.

This aim and these and other objects that will become better apparenthereinafter are achieved by a method for improving the strengthcharacteristics of orthopedic prostheses, such as femur necks, femoralstems, cotyloid elements, system of the spinal column and the like,comprising the step of:

starting from a prosthesis made of metal, subjecting said prosthesis toa peening treatment to improve the surface hardness of the prosthesisand eliminate micro-scoring caused by the machining of machine tools,

characterized in that it comprises a subsequent step of performing asurface finishing treatment by vibration finishing.

Further characteristics and advantages of the invention will becomebetter apparent from the description of a preferred but not exclusiveembodiment of the method according to the present invention.

The method according to the invention provides, starting from anorthopedic prosthesis, such as a femur neck, a femoral stein, a cotyloidelement, a system of the spinal column or a similar prosthesis, for theapplication of a treatment adapted to eliminate substantially themicro-scoring that is present on the surface of the prosthesis due tomachining performed with machine tools.

This treatment is thus aimed at eliminating the micro-scoring that mightgive rise to cracks that lead to failure of the prosthesis and atincreasing the surface hardness of the metal.

The method provides for subjecting the prosthesis to a peeningtreatment, i.e., to a method by means of which the metallic surface ofthe prosthesis is struck (hammered), with a specific intensity, in orderto harden the surface layer and at the same time eliminate the traces ofthe machining of cutting tools.

The peening treatment entails the use of shot that can be made of metal,ceramics, glass or other material. The size of the shot varies and ingeneral its diameter is a fraction of a millimeter. The intensity thatdefines the force with which the shot “hammers” the surface to betreated is also variable and is defined according to the curvature ofspecimens of known thicknesses.

Conveniently, the diameters of the shot are comprised between 0.21 mmand 0.30 mm

The treatment is performed for example with a 200% coverage of thesurface to be treated.

The intensity of the treatment is comprised for example between 4 and 6Almen.

Therefore, the peening treatment to which orthopedic prostheses aresubjected allows hardening of the metallic surface of the prosthesis andat the same time elimination of the micro-scoring that is the cause ofcracks and thus failures of the prosthesis.

Moreover, the hardening of the external surface decreases “frettingcorrosion”, a factor that also causes the initiation of cracks.

Moreover, the peening treatment, besides eliminating the machiningtraces of the tool and increasing the surface hardness of theprosthesis, allows the creation of a prestressed surface layer thatmakes it possible to reduce part of the traction stresses, the highestvalues of which are, as is known, concentrated at the surface. Byreducing surface traction stresses one reduces the likelihood ofinitiation of fracture cracks and thus the likelihood of failure of theprosthesis.

The peening treatment may be performed both on titanium prostheses andon prostheses made of other materials.

After the peening treatment, the prosthesis is subjected to a surfacefinishing treatment provided by vibration finishing.

In particular, the treatment cited above provides for causing thecollision, at different speeds, of the prostheses being worked with anabrasive body (generally defined as “medium”) and for repeating thistype of collision for a sufficient number of times, so that the resultobtained locally by the single collision event can become a generalcharacteristic of the entire surface of the prosthesis.

The vibration finishing treatment is performed in appropriate tanks inwhich the prostheses to be treated and a necessary quantity of mediumare placed. The quantities of medium are selected so as to optimize thenumber of collisions that will define the surface of the finishedprosthesis.

In particular, the prostheses are arranged in a circular vibrationmachine with conical and double wedge-shaped polyester media thatcontain suitable abrasive fillers.

The machining has the goal of eliminating material from the surface ofthe prosthesis in order to reach the desired and necessary roughnessvalue. Surface finishing thus makes it possible to buff the prosthesispreviously subjected to peening, eliminating all the remainingroughnesses and indentations and at the same time also the residues ofthe craters generated by peening.

The surface finishing work is performed by a wet method, with acontinuous input of water and of a suitable detergent. The duration ofthe process is about 16/19 hours.

Subsequently, the prostheses are subjected to a polishing treatment, bymeans of a circular vibration machine with porcelain media (nonabrasive)of spherical shape and having a diameter of for example 4 mm.

The purpose of the polishing step is to modify the roughness parametersof the surface of the prosthesis. This step, too, is performed by a wetmethod with the addition of a polishing product. The duration of thislast step is about 2/3 hours.

In practice it has been found that the method according to the presentinvention fully achieves the intended aim and objects, since it allowsimproving the strength characteristics of prostheses of the known typewith a treatment that is simple, economic and effective and at the sametime it allows obtaining a prosthesis surface with optimum finishing.

The method thus conceived is susceptible of numerous modifications andvariations, all of which are within the scope of the appended claims;all the details may further be replaced with other technicallyequivalent elements.

The disclosures in Italian Patent Application No. MI2010A001762 fromwhich this application claims priority are incorporated herein byreference.

1-8. (canceled)
 9. A method for improving the strength characteristicsof orthopedic prostheses, such as femur necks, femoral stems, cotyloidelements, systems of the spinal column and the like, comprising the stepof: starting from a prosthesis made of metal, subjecting said prosthesisto a peening treatment to improve the surface hardness of the prosthesisand eliminate micro-scoring caused by the machining of machine tools,further comprising a subsequent step of performing a surface finishingtreatment by vibration finishing.
 10. The method according to claim 9,wherein said vibration finishing treatment is performed by subjectingsaid prosthesis to repeated collisions with abrasive means.
 11. Themethod according to claim 10, wherein said abrasive means are polyestermeans.
 12. The method according to claim 9, further comprising asubsequent polishing step.
 13. The method according to claim 12, whereinsaid polishing step is provided by way of nonabrasive porcelain means.14. The method according to claim 12, wherein said polishing step isprovided by means of a circular vibration machine.
 15. The methodaccording to claim 9, wherein said prosthesis is made of titanium. 16.The method according to claim 9, wherein said peening treatment isprovided by means of shot selected from metal, ceramics, glass or othermaterial.